Cold ambient temperatures frequently cause problems in the operation of internal combustion engines, especially diesel engines. It is known that pre-heating the fuel for such an engine, either before carburetion or injection, is useful in improving operation and engine fuel economy. Various fuel pre-heaters for pre-heating fuel using hot coolant from the engine cooling system have been proposed. In general, a fuel heat exchanger of this type could be of various designs, however size and weight have become a restrictive factor in the engine compartments of the newer vehicles. It therefore becomes important to reduce these physical parameters and to improve the efficiency of the heat transfer in such a unit. The present invention aims at such improvements.